In order to establish a noninvasive method of monitoring immediate hypersensitivity reactions in children, we studied the diurnal variation of urinary histamine and 1-methylhistamine excretion and the influence of food intake in a group of 14 healthy nonatopic children (aged 2-16 years). Histamine and 1-methylhistamine in spontaneous urine samples were determined by radioimmunoassay. Mean variation of 2-h intervals was much higher for urinary histamine than for 1-methylhistamine (45% of base-line level versus 24%). There was no circadian rhythm or influence of food intake. The short half-life of histamine released into blood circulation may be the main reason for the higher variation of histamine excretion. In children, urinary 1-methylhistamine is less influenced by diurnal variation and is therefore more suited to monitor immediate hypersensitivity reactions than urinary histamine itself.